1. Field of the Invention
The present invention relates to mobile communication systems.
2. Introduction
A mobile relay station (MRS) is a relay station that is capable of handover between base stations (BS) (or relay stations for above 3-hops). It is usually attached to a public transportation system (e.g., a bus, train, etc.) and may relay information to/from mobile stations (MSs) inside the public transportation. When MRS makes handover from BS1 to BS2, MRS should take care for MSs under communication via the MRS. Due to handover process of MRS, synchronization between MRS and MSs may be lost. It leads to a communication disruption.
If MS makes handover as same as conventional cellular system, it would be no problem for MS to prepare handover process and make a decision. When RS makes handover, the RS must be cognizant of MSs under communication via the RS. Synchronization could be an issue between RS and MSs when RS makes handover between BSs. A problem may occur in the change of timing of preamble and broadcast message that is transmitted from RS to MSs. If the MSs lose synchronization, it takes time to recover it. During the recovery process, communication with MSs may be suspended. The synchronization process of MSs consumes radio resources to reestablish the synchronization. If there are many MSs under the RS, the overhead amounts may be large.
If the RS can maintain the same preamble and broadcast timing when the RS makes a handover between BSs, this problem does not occur. However, it would be difficult to assign same preamble and broadcast timing after the RS has completed the handover as before the handover because preambles and following broadcasts may interfere. The information exchange between BSs via the backbone network would be too complicated to be performed in a practical manner.